As described in Clin. Microbiol. Rev., 1994, pp. 43-54, there are many attempts to separate a variety of biological molecules and microorganisms by immobilizing antibodies or pairing bases to magnetic fine particles. As the magnetic fine particles for use in these methods, those having a particle size of 1 μm or more are usually used in consideration of the time for recovering them with a magnet. However, since the surface area decreases as the particle size increases, the efficiency is a big problem in the case that a small molecule such as a protein or nucleic acid is tried to separate.
As a method for solving the problem of particle size, Applied. Microbiol. Biotechnol., 1994, Vol. 41, pp. 99-105 and Journal of fermentation and Bioengineering, 1997, Vol. 84, pp. 337-341 have reported that magnetic fine particles having a lower critical solution temperature (hereinafter referred to as “LCST”) and a particle size of about 100 to 200 nm are obtained by polymerizing polyisopropylacrylamide having an LCST in the presence of magnetic fine particles and that the magnetic fine particles is possible to recover easily through their aggregation caused by raising the solution temperature to the LCST or higher even though the particle size of the magnetic fine particles is smaller than that of conventional particles.
In the reports, methacrylic acid is copolymerized at the polymerization of polyisopropylacrylamide, bovine serum albumin is then immobilized to the carboxylic acid using a carbodiimide after completion of the polymerization, and purification of bovine serum albumin in serum is attempted.
However, this method requires immobilization of an enzyme, an antibody, or the like to the polymer on the magnetic particles depending on individual intended uses and also requires removal of unreacted substances, so that it is necessary to carry out operations for purification and hence the method is not versatile. Moreover, in the method, since a nucleic acid should be directly bound to a protein or the like, the immobilization of a nucleic acid is very difficult.
As described above, in the conventional technology, the usefulness of the magnetic fine particles having an LCST is recognized in the field of separating agents and the like, but there is a problem of poor versatility.